Tuesday, October 02, 2018

Creating and Using Tokens with Azure Service Bus

The following notes are with regards to the “.net standard” Microsoft.Azure.ServiceBus library and details what each parameter is (because I found the documentation lacking in this regard).

Creating a token

Tokens are created using a TokenProvider. There are many options in instantiating a token provider and then one I am showing here utilizes the Shared Access Signature (SAS). But, if you are running your code in Azure, a better option would be to use the ManagedServiceIdentityProvider, as one doesnt need to share keys, etc.

To create a token using the “CreateSharedAccessSignatureTokenProvider(string keyName, string sharedAccessKey)”, you need a key name and its key. This is retrieved from the “Shared access policies” blade in Azure. Once you have these 2 values, you can create a tokenProvider.

TokenProvider tokenProvider = TokenProvider.CreateSharedAccessSignatureTokenProvider(keyName, sasKey);

The CreateSharedAccessSignatureTokenProvider has overloads, in which you can specify the TTL of the token and the scope (namespace vs entity).

To create the token, you need to use the “GetTokenAsync(string appliesTo, TimeSpan timeout)” on the tokenProvider. The “appliesto” defines the either the namespace or the entity and should be defined as follows:

For access to the entire namespace: testservicebusinstance.servicebus.windows.net
For access to an entity in the namespace called myFirstQueue: testservicebusinstance.servicebus.windows.net/myFirstQueue

Using the token

The GetTokenAsync method returns a token. The token contains a property called TokenValue. You use this TokenValue to work with the servicebus.

var tokenProvider = TokenProvider.CreateSharedAccessSignatureTokenProvider(token.TokenValue);
var client = new QueueClient("testservicebusinstance.servicebus.windows.net", "myFirstQueue", tokenProvider);
await client.SendAsync(message);

Controlling of access to the service bus:

There are 2 things that are possible:
1. restrict scope to the entity
2. specify if the token can manage,send or listen

The first one, I showed you in “creating a token”. The second one is not so obvious and is controlled by the Claims on the SaS policy that you are using (the keyName and sasKey for which you pulled from the SAS Policies blade in Azure).

When to use tokens:

Say you have a lot of different micro-services and they all need to get access the ServiceBus. Also, you dont want to give each one a key, as you now will have to figure out what to do if the key got compromised. Also, you dont want all services to be able to do anything and everything on the ServiceBus. In this case, you would need to stand up a token provider service. The microservices, would then authenticate with the token service and the token service would then determine what the microservice is authorized to do and return a token that has the appropriate claims and scope.

Best practices:

As you can see with the SharedAccessSignature option, you still need to provide your token service the sas-key. Instead, with the release of managed service identities support on ASB, one can do this without the sas-key. So, if your service is going to be hosted in Azure, then MSI is definitely the way to go.

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